British soldiers and airmen who helped to operate a secretive US detention facility in Baghdad that was at the centre of some of the most serious human rights abuses to occur in Iraq after the invasion have, for the first time, spoken about abuses they witnessed there.

Personnel from two RAF squadrons and one Army Air Corps squadron were given guard and transport duties at the secret prison, the Guardian has established.

And many of the detainees were brought to the facility by snatch squads formed from Special Air Service and Special Boat Service squadrons.

Codenamed Task Force 121, the joint US-UK special forces unit was at first deployed to detain individuals thought to have information about Saddam Hussein's weapons of mass destruction. Once it was realised that Saddam's regime had long since abandoned its WMD programme, TF 121 was re-tasked with tracking down people who might know where the deposed dictator and his loyalists might be, and then with catching al-Qaida leaders who sprang up in the country after the regime collapsed.

Suspects were brought to the secret prison at Baghdad International airport, known as Camp Nama, for questioning by US military and civilian interrogators. But the methods used were so brutal that they drew condemnation not only from a US human rights body but from a special investigator reporting to the Pentagon.

A British serviceman who served at Nama recalled: "I saw one man having his prosthetic leg being pulled off him, and being beaten about the head with it before he was thrown on to the truck."

On the 10th anniversary of the invasion of Iraq, a number of former members of TF 121 and its successor unit TF6-26 have come forward to describe the abuses they witnessed, and to state that they complained about the mistreatment of detainees.

The abuses they say they saw include:

• Iraqi prisoners being held for prolonged periods in cells the size of large dog kennels.

• Prisoners being subjected to electric shocks.

• Prisoners being routinely hooded.

• Inmates being taken into a sound-proofed shipping container for interrogation, and emerging in a state of physical distress.

It is unclear how many of their complaints were registered or passed up the chain of command. A Ministry of Defence spokesperson said a search of its records did not turn up "anything specific" about complaints from British personnel at Camp Nama, or anything that substantiated such complaints.

Nevertheless, the emergence of evidence of British involvement in the running of such a notorious detention facility appears to raise fresh questions about ministerial approval of operations that resulted in serious human rights abuses.

Geoff Hoon, defence secretary at the time, insisted he had no knowledge of Camp Nama. When it was pointed out to him that the British military had provided transport services and a guard force, and had helped to detain Nama's inmates, he replied: "I've never heard of the place."

The MoD, on the other hand, repeatedly failed to address questions about ministerial approval of British operations at Camp Nama. Nor would the department say whether ministers had been made aware of concerns about human rights abuses there.

Former army officer Crispin Blunt accused defence secretary John Hutton in 2009 of sweeping under the carpet the evidence of direct British service involvement. Photograph: Anthony Devlin/PA

However, one peculiarity of the way in which UK forces operated when bringing prisoners to Camp Nama suggests that ministers and senior MoD officials may have had reason to know those detainees were at risk of mistreatment. British soldiers were almost always accompanied by a lone American soldier, who was then recorded as having captured the prisoner. Members of the SAS and SBS were repeatedly briefed on the importance of this measure.

It was an arrangement that enabled the British government to side-step a Geneva convention clause that would have obliged it to demand the return of any prisoner transferred to the US once it became apparent that they were not being treated in accordance with the convention. And it consigned the prisoners to what some lawyers have described as a legal black hole.

Surrounded by row after row of wire fencing, guarded by either US Rangers or RAF personnel, and with an Abrams tank parked permanently at its main gate, to the outside observer Camp Nama seemed identical to scores of military bases that sprang up after the 2003 invasion of Iraq. Once inside, however, it was clear that Nama was different.

Not that many people did enter the special forces prison. It was off limits to most members of the US and UK military, with even the officer commanding the US detention facility at Guantánamo being refused entry at one point. Inspectors from the International Committee of the Red Cross were never admitted through its gates.

General Stanley McChrystal, then commander of US Joint Special Operations forces in Iraq, was said to have visited Nama. Photograph: Christian Sinibaldi for the Guardian

While Abu Ghraib prison, just a few miles to the west, would achieve global notoriety after photographs emerged depicting abuses committed there, Camp Nama escaped attention for a simple reason: photography was banned. The only people who attempted to take pictures – a pair of US Navy Seals – were promptly arrested. All discussion of what happened there was forbidden.

Before establishing its prison at Nama, TF 121 had been known as Task Force 20, and had run a detention and interrogation facility at a remote location known as H1, in Iraq's western desert. At least one prisoner had died en route to H1, allegedly kicked to death aboard an RAF Chinook.

The British were always junior partners in TF 121. Their contingent was known as Task Force Black. US Delta Force troops made up Task Force Green and US Army Rangers Task Force Red. One half of Task Force Black comprised SAS and SBS troopers, based a short distance away at the government compound known as the Green Zone. They detained so-called high-value detainees, who were brought to Camp Nama. The other half were the air and ground crews of 7 Squadron and 47 Squadron of the RAF, and 657 Squadron of the Army Air Corps, who lived on the camp itself, operating helicopters used in detention operations and a Hercules transport aircraft.

"The Americans went out to bring in prisoners every night, and British special forces would go out once or twice a week, almost always with one American accompanying them," one British serviceman who served at Nama recalled earlier this month.

''The prisoners would be brought in by helicopter, usually one at a time, although I once saw five being led off a Chinook. They were taken into a large hangar to be bagged and tagged, a bag put over their heads and their hands plasticuffed behind their backs. Then they would be lifted or thrown on to the back of a pick-up truck and driven to the Joint Operations Centre."

The Joint Operations Centre, or JOC, was a single storey building a few hundred yards from the airport's main runway. Some of those who served at Nama believed it had formerly been used by Saddam's intelligence agencies.

The US and UK forces worked together so closely that they began to wear items of each others' uniforms. But while British personnel were permitted into the front of the JOC, few were allowed into the rear, where interrogations took place. This was the preserve of US military interrogators and CIA officers based at Camp Nama. "They included a number of women," said one British airman. "One had a ponytail and always wore two pistols, so we had to nickname her Lara Croft."

There were four interrogation cells at the rear of the JOC, known as the blue, red, black and soft rooms, as well as a medical screening area. The soft room contained sofas and rugs, and was a place where detainees could be shown some kindness. Harsh interrogations took place in the red and blue rooms, while the black room – described as windowless, with hooks in the ceiling, and where every surface was painted black – is said to be the cell where the worse abuses were perpetrated.

According to an investigation by Human Rights Watch, the New York-based NGO, detainees were subject to "beatings, exposure to extreme cold, threats of death, humiliation and various forms of psychological abuse or torture" at the JOC. The New York Times has reported that prisoners were beaten with rifle butts and had paintball guns fired at them for target practice.

Signs posted around Nama are said to have proclaimed the warning "No Blood, No Foul": if interrogators did not make a prisoner bleed, they would not face disciplinary action.

There was also an overspill interrogation room cell behind the JOC: a shipping container lined with padding. "You could see people being taken in there, and they were in pretty poor shape when they were taken out," said one British witness. He adds: "Everyone's seen the Abu Ghraib pictures. But I've seen it with my own eyes."

A number of British soldiers who served with TF 121 said that some SAS officers were permitted to attend interrogations at the rear of the JOC. Human Rights Watch reports that one SAS officer took part in the beating of a prisoner thought to know the whereabouts of Abu Musab al-Zarqawi, the leader of al-Qaida in Iraq.

While not being interrogated, according to witnesses, prisoners were held in cells the size of large dog kennels. "They were made of wire mesh with sloping corrugated roofs," said a British ex-serviceman who served at Nama. "They were chest high, and two feet wide. There were about 100 of them, in three rows, and they always appeared to have at least one prisoner in each. They would be freezing at night, and really hot during the day.

"The prisoners were mostly men, although I did see two women being taken into the JOC for interrogation. I've no idea what became of them, or to any of the male prisoners after their interrogation was completed."

Some of the scenes at Nama were so disturbing that personnel serving there would literally look the other way, rather than witness the abuse. "I remember being on sentry duty at a post overlooking the dog kennels, and the guy I was with wouldn't even look at them," one British eyewitness recalls. "I was saying: 'Hey turn around and look at them.' And he wouldn't. He just wouldn't turn around, because he knew they were there."

Some complaints made at the time by British personnel were immediately suppressed. "I remember talking to one British army officer about what I had seen, and he replied: 'You didn't see that – do you understand?' There was a great deal of nervousness about the place. I had the impression that the British were scared we would be kicked off the operation if we made a fuss," the ex-serviceman said.

According to one US interrogator interviewed by Human Rights Watch, however, written authorisations were required for many of the abuses inflicted on prisoners at Nama, indicating that their use was approved up the chain of command.

"There was an authorisation template on a computer, a sheet that you would print out, or actually just type it in," the interrogator said. "It was a checklist. It was already typed out for you, environmental controls, hot and cold, you know, strobe lights, music, so forth. But you would just check what you want to use off, and if you planned on using a harsh interrogation you'd just get it signed off. It would be signed off by the commander."

According to one British serviceman who was at Nama, US soldiers would bring prisoners in every night. Photograph: Jehad Nga/Corbis

Camp Nama was such a secret location that when General Geoffrey Miller, the commander of the detention centre at Guantanamo Bay, was sent to Iraq in August 2003 to advise on interrogation regimes he was initially refused entry, according to Human Rights Watch.

At the end of 2003, the Pentagon sent a special investigator, Stuart Herrington, a retired military intelligence colonel, to discover more about the methods being employed at Nama. In December that year Herrington reported: "Detainees captured by TF 121 have shown injuries that caused examining medical personnel to note that 'detainee shows signs of having been beaten'. It seems clear that TF 121 needs to be reined in with respect to its treatment of detainees."

More than 30 members of the task force were subsequently disciplined for abusing prisoners. Yet the beatings continued, according to British witnesses. The dog kennel cells remained in place, and UK special forces continued to be used to snatch suspects to be brought in for interrogation. "I can see now that we were supplying the meat for the American interrogators," says one.

In February 2004, senior British special forces and intelligence officers felt emboldened enough to mount a detention operation without an accompanying US soldier. Troopers surrounded a house in southern Baghdad that MI6 had identified as a safe house for foreign fighters. Two men were killed in the raid and two others of Pakistani origin were detained and handed over to the US authorities.

After questioning at Nama, the pair were flown to Bagram, north of the Afghan capital, Kabul, where they are thought to remain incarcerated, despite efforts by lawyers to secure their release by persuading the appeal court in London to order the issuing of a writ of habeas corpus.

Two months later, in April 2004, US news media published a series of shocking photographs showing the abuse of prisoners at a different prison, Abu Ghraib, where individuals detained by regular troops rather than special forces were being held. A few days later Task Force 121 was renamed Task Force 6-26. Shortly after this, two US Navy Seals – who had their own compound with Camp Nama – were seen taking photographs from the roof of their building. Both men were immediately arrested, British witnesses say and were not seen at Nama again.

Later that summer the secret prison was moved to Balad, a sprawling air base 50 miles north of Baghdad, where it became known as the Temporary Screening Facility (TSF). The Army Air Force and RAF troops continued their role there.

SAS troops continued to provide detainees for interrogation, operating from their base in one of a row of seven large villas inside the Green Zone. The villa next door was occupied by troops from Delta Force. Each of the homes had a swimming pool, and at the end of the long garden behind the SAS villa was a large hut occupied by a UK military intelligence unit, the Joint Forward Interrogation Team, or JFIT.

Individuals detained by the SAS – accompanied by their lone American escort – would be flown by helicopter to a landing pad behind the villas, and taken straight to the JFIT. According to former members of TF 6-26, after a brief interrogation by the British, they would be handed over to US forces, who would question them further before releasing them, or arrange for them to be flown north to Balad.

In late 2003, according to former taskforce members, two SAS members wandered next door to the Delta Force villa, where they were horrified to see two Iraqi prisoners being tortured. "They were being given electric shocks from cattle prods and their heads were being held under the water in the swimming pool. There were less visits next door after that."

While a complaint was made, it is not thought to have reported through the chain of command. And it certainly appears not to have reached Downing Street, as shortly afterwards Tony Blair, then prime minister, visited the SAS house to thank the troopers for their efforts.

By the end of 2004, according to the BBC journalist Mark Urban, MI6 officers who had visited the secret prison at Balad were expressing concern that the kennel cells had been reconstructed there, and the British government later warned the US authorities that it would hand over prisoners only if there was an undertaking that they would not be sent there.

Shortly afterwards, the RAF Hercules operated by the task force was shot down while flying from Nama to Balad, with the loss of all 10 men on board. It was the largest loss of life suffered by the RAF in a single incident since the second world war.

By now, a growing number of British members of the task force were deeply disillusioned about their role. When one, SAS trooper Ben Griffin, decided he could not return to Iraq, he expected to be face a court martial. Instead, he discovered that a number of his officers sympathised with him, and he was permitted to leave the army with a first-class testimonial.

When Griffin went public, making clear that British troops were handing over to the US military large numbers of prisoners who faced torture, the MoD came under pressure to explain itself. In February 2009 the then defence secretary, John Hutton, told the Commons that "review of records of detention resulting from security operations carried out by UK armed forces" had disclosed that two men who had been handed over had since been moved to Afghanistan. His statement made no mention of the joint task force, of H1, or of Camp Nama or Balad or how British airmen and soldiers were helping to operate the secret prisons.

Crispin Blunt, a Tory MP and former army officer, accused Hutton of "simply sweeping under the carpet the apparent evidence of direct British service involvement with delivery to gross mistreatment amounting to torture involving hundreds if not thousands of people".

Today, 10 years after the invasion and the creation of the joint US-UK taskforce that detained and interrogated large numbers of Iraqis, the MoD responds to questions about their abuse by stating that it is aware only of "anecdotal accounts" of mistreatment, and that "any further evidence of human rights abuse should be passed to the appropriate authorities for investigation".

Griffin had done just that, asking the MoD itself to investigate the activities of the taskforce of which he had been a member. The MoD obtained an injunction to silence him, and warned he faced jail if he ever spoke out again.